linux-kernel - Re: [PATCH 1/2] KVM: VMX: Bury Intel PT virtualization (guest/host mode) behind CONFIG

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Message-ID: <448c8367-9f54-4ab1-80c3-bb13c9ac4664@intel.com>
Date: Mon, 4 Nov 2024 10:23:17 +0200
From: Adrian Hunter <adrian.hunter@...el.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] KVM: VMX: Bury Intel PT virtualization (guest/host
 mode) behind CONFIG_BROKEN

On 1/11/24 20:50, Sean Christopherson wrote:
> Hide KVM's pt_mode module param behind CONFIG_BROKEN, i.e. disable support
> for virtualizing Intel PT via guest/host mode unless BROKEN=y.  There are
> myriad bugs in the implementation, some of which are fatal to the guest,
> and others which put the stability and health of the host at risk.
> 
> For guest fatalities, the most glaring issue is that KVM fails to ensure
> tracing is disabled, and *stays* disabled prior to VM-Enter, which is
> necessary as hardware disallows loading (the guest's) RTIT_CTL if tracing
> is enabled (enforced via a VMX consistency check).  Per the SDM:
> 
>   If the logical processor is operating with Intel PT enabled (if
>   IA32_RTIT_CTL.TraceEn = 1) at the time of VM entry, the "load
>   IA32_RTIT_CTL" VM-entry control must be 0.
> 
> On the host side, KVM doesn't validate the guest CPUID configuration
> provided by userspace, and even worse, uses the guest configuration to
> decide what MSRs to save/load at VM-Enter and VM-Exit.  E.g. configuring
> guest CPUID to enumerate more address ranges than are supported in hardware
> will result in KVM trying to passthrough, save, and load non-existent MSRs,
> which generates a variety of WARNs, ToPA ERRORs in the host, a potential
> deadlock, etc.
> 
> Fixes: f99e3daf94ff ("KVM: x86: Add Intel PT virtualization work mode")
> Cc: stable@...r.kernel.org
> Cc: Adrian Hunter <adrian.hunter@...el.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 6ed801ffe33f..087504fb1589 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -217,9 +217,11 @@ module_param(ple_window_shrink, uint, 0444);
>  static unsigned int ple_window_max        = KVM_VMX_DEFAULT_PLE_WINDOW_MAX;
>  module_param(ple_window_max, uint, 0444);
>  
> -/* Default is SYSTEM mode, 1 for host-guest mode */
> +/* Default is SYSTEM mode, 1 for host-guest mode (which is BROKEN) */
>  int __read_mostly pt_mode = PT_MODE_SYSTEM;
> +#ifdef CONFIG_BROKEN
>  module_param(pt_mode, int, S_IRUGO);
> +#endif

Side effects are:
1. If pt_mode is passed via modprobe, there will be a warning in kernel messages:
	kvm_intel: unknown parameter 'pt_mode' ignored
2. The sysfs module parameter file pt_mode will be gone:
	# cat /sys/module/kvm_intel/parameters/pt_mode
	cat: /sys/module/kvm_intel/parameters/pt_mode: No such file or directory

Nevertheless:

Tested-by: Adrian Hunter <adrian.hunter@...el.com>

>  
>  struct x86_pmu_lbr __ro_after_init vmx_lbr_caps;
>